1. Field of the Invention
This invention relates to a semiconductor device formed by sealing at least a semiconductor element thermally connected to a radiator plate with a molded resin, a method of fabricating the semiconductor device and an apparatus for fabricating the semiconductor device, or in particular to a semiconductor device formed by sealing at least a semiconductor element sandwiched between a pair of radiator plates with a molded resin, a method of fabricating the semiconductor device and an apparatus for fabricating the semiconductor device.
2. Description of the Related Art
FIG. 7 is a schematic diagram showing a sectional structure of an ordinary resin molded semiconductor device.
Generally, in the semiconductor of this type, as shown in FIG. 7, the two surfaces of a semiconductor element 10 are sandwiched by radiator plates 20, 30 and the resulting assembly is sealed with a molded resin 80 so as to expose the radiation surfaces 20a, 30a of the radiator plates 20, 30. In other words, the parts of the semiconductor device other than the radiation surfaces 20a, 30a of the radiator plates 20, 30 are sealed with the molded resin 80.
The heat generated from the active semiconductor element 10 is transmitted to the upper and lower radiator plates 20, 30 and radiated out of the semiconductor device from the radiation surfaces 20a, 30a of the radiator plates 20, 30.
The radiation surfaces 20a, 30a of the radiator plates 20, 30 are the widest surfaces of the radiator plates 20, 30, i.e. the outer main surfaces 20a, 30a of the pair of the radiator plates 20, 30 sandwiching semiconductor element 10.
In the semiconductor device shown in FIG. 7, the semiconductor element 10 is sandwiched between the pair of the radiator plates 20, 30 through a heat sink block 40 and conductive bonding materials 51 to 53 of such a material as solder or a conductive adhesive.
In the molded resin 80, at least a lead frame 60 is arranged around the semiconductor element 10, which is electrically connected to each lead frame 60 through a wire 70.
This semiconductor device can be fabricated by molding the semiconductor element 10 with the radiator plates 20, 30 according to the transfer molding process using a resin-sealing die.
FIG. 8 is a sectional view schematically showing the conventional ordinary molding method using the die 200 for the semiconductor device. As shown in FIG. 8a, a cavity 200a is formed in the die 200 by joining an upper die 201 and a lower die 202 each other.
First, the semiconductor element 10 is sandwiched by a pair of the radiator plates 20, 30 and connected to a lead frame 60 through a wire 70. Then, the resulting assembly is set in the cavity 200a of the die 200 and a resin is injected and filled in the die 200, which is thus sealed by the molded resin 80.
In this molding method using resin, as shown in FIG. 8a, the tolerance of the work thickness generally requires a clearance K to be formed between the upper die 201 and the radiation surface 30a. 
At the time of molding, resin flows into the clearance K. As a result, as shown in FIG. 8b, the radiation surface 30a of the upper radiator plate 30 of the completed semiconductor device is covered with the mold resin 80.
In other words, a resin burr can be easily formed on the radiation surface 30a of the radiator plate 30. This is the same on the radiation surface 20a of the lower radiator plate 20. Once the resin burr occurs, the radiation efficiency of the radiation surfaces 20a, 30a is undesirably reduced.
In a conventional method which has been proposed to prevent from forming the resin burr on the radiator plates 20, 30, a heat-resistant, a flexible material is inserted between the die and the radiation surfaces to fill the clearance and prevents the intrusion of the mold resin (Japanese Patent No. 3350444).
Also, a method has been proposed to prevent the intrusion of the mold resin onto the radiation surfaces by bonding the radiation surfaces of the radiator plates to the die and thus eliminating the clearance (Japanese Unexamined Patent Publication No. 10-223669).
Furthermore, with the semiconductor device having such a configuration that at least a semiconductor element is sandwiched by a pair of radiator plates and heat is radiated from the two surfaces of the semiconductor element, the radiation surface of one radiator plate is likely to be tilted relative to the radiation surface of the other radiator plate. Therefore, it is difficult to hold the two radiation surfaces parallel to each other.
A semiconductor device including a semiconductor element sandwiched by a pair of radiator plates and sealed by the molded resin, has normally a cooling structure in which a cooling member is arranged in contact with each radiation surface of the radiator plate pair so as to sandwich the semiconductor device.
The radiation surfaces are cooled with the cooling members to speed up the heat radiation from the radiation surfaces.
In this case, however, a poor parallelism between the radiation surfaces of the radiator plates would lead to a gap and an insufficient contact between each radiation surface and the corresponding cooling member, thereby often deteriorating the heat radiation efficiency.
The deterioration of the parallelism between the radiation surfaces is conspicuous in many cases where a plurality of semiconductor elements are sandwiched between a pair of radiator plates. This is due to the often irregular thickness of the semiconductor elements which makes it difficult to maintain the parallelism between the radiation surfaces.
The present inventors have tried to secure a satisfactory parallelism between the radiation surfaces of the radiator plates by cutting or grinding the radiation surfaces and thus adjusting the parallelism between the radiation surfaces after sealing the device with the molded resin.
In the conventional semiconductor device, as shown in FIG. 7, the side surfaces of the radiator plates 20, 30 are sealed with the molded resin although the radiation surfaces 20a, 30a of the radiator plates 20, 30 are exposed from the mold resin 80.
When the radiation surfaces 20a, 30a are cut or ground with a machining member such as a cutting tool or a grinding stone, therefore, the risk of the machining member cutting the very hard molded resin 80 together with the radiator plates 20, 30 results in a very great consumption of the machining member. Thus, the life of the machining member is considerably shortened and the cost of machining the radiation surfaces becomes higher.
The radiation surfaces may be machined by shot blasting without using any machining member such as the cutting tool or the grinding stone. The shot blasting is not undesirable, however, because of the need of the cleaning and drying processes after machining and a higher cost of materials such as abrasives.
The aforementioned problem of the great consumption and the shorter life of the machining member for cutting or grinding the radiation surfaces is not limited to the semiconductor device in which the two surfaces of the semiconductor element are sandwiched by a pair of radiator plates and the assembly is sealed with the molded resin while exposing the heat radiation surface of each radiator plate.
In other words, the problem of the machining member described above is shared by all the semiconductor devices in which the semiconductor element is thermally connected with the radiator plates and the resulting assembly is sealed with the molded resin so as to expose the radiation surfaces of the radiator plates.